- With the rack in the upper third of the oven, preheat the oven’s broiler.
- In an ovenproof 23 cm (9-inch) non-stick skillet, brown the onion and garlic in 30 ml (2 tbsp) of the oil. Let cool in a bowl. In the same skillet, soften the lettuce. Transfer to a plate.
- Whisk the eggs and Parmesan into the onion mixture. Season with salt and pepper. Set aside.
- In the same skillet, heat the remaining oil (30 ml / 2 tbsp). Fill with the egg mixture. Cook over medium heat, stirring gently with a spatula for about 30 seconds. Spread the wilted lettuce over the entire surface. Cook for about 5 minutes or until the edges of the frittata begin to brown.
- Transfer to the oven, and finish cooking for about 5 minutes or until the top of the frittata is set and golden brown.
- Serve the frittata hot or warm.